Tension member position regulating system utilizing a variable limiter



Dec. 31, 1968 J. M. BENTLEY ET AL 3;419,771 TENSION MEMBER POSITION REGULATING SYSTEM UTILIZING A VARIABLE LIMITER Filed March 29. 1966 mmri isz 29.538-

INVENTORS John M. Bentley and Wesley H. Watson United States Patent 3,419,771 TENSION MEMBER POSITION REGULATING SYSTEM UTILIZING A VARIABLE LIMITER John M. Bentley, East Aurora, and Wesley H. Watson,

Williamsville, N.Y., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 29, 1966, Ser. No. 538,269 17 Claims. (Cl. 318-6) ABSTRACT OF THE DISCLOSURE In a system wherein a swing roll position controller produces roll position error signal that is fed to a speed regulator, means is provided for initially limiting the output of the position controller to a small value regardless of roll position, and further means is provided for removing that limitation in response to the swing roll moving within close range of a reference position, whereby transition from speed to position modes is smoothly effected without severe transients.

This invention relates to apparatus for controlling the position of a movable member whose departure from a norm or reference position is correctable by controlling the speed of traveling elongate material. More particularly, the invention is directed to apparatus for controlling the position of a tension member that is in contact with a motor driven traveling strip, for example a biased swing roll for tensioning paper strip in paper processing apparatus.

In paper processing it is desirable to control the sheet tension on paper strip as it travels from section to section of a paper process line, for example between stacks of calendar rolls, between pull rolls and coater sections, etc. Swing rolls are employed as a convenient means of sensing sheet tension. By sensing swing roll position and maintaining a reference position, tension can indirectly be controlled. The tension, however, is actually established, and adjusted, by means of resilient loading on the swing roll pivot arms, for example, pneumatic loading, spring loading, etc. In a particular system, a swing roll position signal generator generates a position error signal which is fed into the speed controller of a speed regulating loop to either add or subtract from the command or reference speed signal. This corrective signal from the position signal generator causes the speed regulating loop to regulate for a new speed until the swing roll has been returned to its reference position, for example center. In prior systems in order to close the swing roll position regulating loop without introducing transients severe enough to break the paper, it was necessary to first maneuver the swing roll near its reference position (for example center) either by direct manual mechanical movement or by careful and delicate speed control. Such an operation is subject to human error and is also wasteful of time, especially if it is desired to switch from speed to position control with the swing roll at one of its extreme travel positions.

It is therefore, an object of the present invention to provide an arrangement whereby transition from speed to position control mode in such systems may be smoothly made without severe transients, even if the swing roll is at the extreme position on either side of the reference position when the transition is made.

In accordance with one embodiment of the invention the objects of the invention may be realized in a system wherein the output of tthe swing roll error generator is initially limited to a relatively small value so that a soft transition can be made. The limitation is maintained until the swing roll nears the center or reference position, at

Patented Dec. 31, 1968 which time and in response to the swing roll having attained this position, the limitation is removed or lifted to allow full position regulating capability of the system.

The above and other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the drawings which illustrate a system incorporating a preferred embodiment of the invention.

In the drawings:

FIGURE 1 is a diagram of a swing roll position regulator incorporating the invention; and

FIG. 2 is a graph illustrating the relationship between transducer output and swing roll position.

The system shown in the drawing provides control for strip material 10 for example paper strip traveling in the direction of the arrow 12 along a course between sections in a paper processing machine. A set of rolls 14 may be merely guide rolls or they may be drive or press rolls driven by a motor and regulator therefore (not shown). A set of rolls 16 for example press rolls is driven by a motor 18 controlled by a regulator 20.

Regulated tension is applied to the strip 10 by a swing roll 22 rotatably mounted at the lower end of a pivot arm 24 keyed to a shaft 26 so journalled that the arm 24 is swingable through an arc in the directions of the double ended arrow 28 between extreme excursion limits defined by fixed stops 30 and 32. The tension is established and adjusted by means of resilient bias or loading applied in the direction of arrow 34 to the arm 24, for example by a pneumatic system 36. In the system 36, a piston 38 operating in an air cylinder 40, is flexibly coupled to arm 24 by means of a sliding pin and slot arrangement 42. Cylinder 40 is connected through a control valve 44 to a suitable air supply source (not shown) which will hold pressure constant at any selected value.

The regulator 20 has three regulating loops, a current regulating inner loop 46, a speed regulating intermediate loop 48 superposed on the inner loop, and a swing roll position regulating outer loop 50, superposed on the intermediate loop. Effectively the loops 50, 48 and 46 are cascaded in that order. The current regulating loop 46 includes a power supply source 52, which responds to a current controller 54, that in turn is responsive jointly to the output of the speed loop 48 and a negative feedback signal that is a function of the current supplied to the motor. The power supply source 52 provides an output to the motor 18 that is a function of an input control parameter. For example the output voltage of the source 52 is proportional to the magnitude of an input control signal supplied by the controller 54. By way of example the power supply 52 is shown as comprising a DC generator 58 having an armature 60 and a main field 62 energized by the output of a power amplifier 64 whose control input circuit is coupled to and in response to the output of controller 54.

Amplifier 64 provides an average DC output voltage that is proportional to control signals applied to its control input line 66, which in this example is connected to the output line of controller 54. Amplifier 64 may be of any suitable type. For example it may have an output stage employing semiconductor controlled rectifiers that are phase controlled by gating circuits in response to input controlled signals supplied on line 66. The generator armature 60 supplies power to output lines 68 and 70 of the power supply source. The generator 58 is shown as being driven by a suitably powered electric motor 72.

The output line 68 is connected to one terminal of the armature 72 of motor 18, while the other output line 70 is connected through a current supplying resistor 174 to the other terminal of armature 72. Motor 18 is provided with a main field 76 connected to a suitable source of power (not shown). Both the generator 58 and the motor 18 may have series field windings which are omitted to simplify the illustration. Motor 18 drives the rolls 16 through a coupling 78. If desired, the power amplifier 64 may be arranged to supply motor 18 directly, in which case the intervening generator 58 is eliminated, and the output lines of amplifier 64 are connected directly to lines 68 and 70. The output of controller 54 is a current error signal representing the difference between commanded current and the actual armature current of motor 18.

A current command signal derived from the output of a speed controller 80 in the speed loop 48 is applied to the summing junction 82 in the input of controller 54 through an input resistor 84. A negative feedback signal proportional to motor armature current is applied through an input resistor 86 to the summing junction 82. The feedback signal is produced by a feedback circuit 88 that includes an isolation amplifier 90 whose output is connected to the input resistor 86, and whose input is taken from across resistor 74. Amplifier 90 effects isolation between input and output and may be of any suitable type for example, a magnetic amplifier. If isolation and/ or amplification are not required, the isolating amplifier may be dispensed with.

The current regulating loop 46 is so arranged that the current command signal supplied to the input resistor 84 and the feedback signal applied to resistor 86 are summed to energize the current controller 54 in such a manner that the armature current in the armature circuit of the motor is regulated to a value which is a function of the signal derived from the output of speed controller 80 and applied to the input resistor 84. Suitable limiting means 91 may be associated with controller 80 to limit the output of controller 80 in order to prevent the motor armature current from going above a predetermined limit.

Besides the controller 80, the speed loop 48 contains the current loop 46 and also includes an adjustable speed reference source 92 and an actual speed negative feedback circuit 94. The output of the speed reference source 92 is coupled to an input resistor 96 connected to the summing junction 98 in the input of controller 80. Source 92, which may be a potentiometer connected across a suitable DC source, supplies to the summing junction 98 a speed reference or command signal that is proportional to a desired (commanded) speed for the strip at the rolls 16.

Feedback circuit 94 applies to the summing junction 98 of controller 80 a signal which is a function of the actual speed of the strip 10 at the rolls .16. The feedback circuit 94 includes a DC tachometer generator 100 mechanically coupled to the motor 18. Since the speed of motor 18 is proportional to the speed of the strip '10 at the rolls 16, the output of tachometer 100 is proportional to the speed of the strip 10 at the set of rolls 16. The output of the tachometer generator 100 is applied through an input resistor 102 to the summing junction 98 in negative feedback relation to the reference speed signal applied to that summing junction through resistor 96 as may be modified by a signal derived from the output of a position controller 104 in the position regulating loop 50 and applied through an input resistor 106 to the summing junction 98 under circumstances hereinafter described. The arrangement of the speed regulating loop 48 and its relation to the current regulating loop 46 is such that the speed of motor 18 is regulated to a value dictated by the speed reference signal as modified by the output signal from controller 104 to the position regulating loop 24.

By way of example, controllers 54, 80 and 104 are shown as inverting bi-directional output operational amplifiers with feedback networks that give these controllers proportional-integral (PI) characteristics.

The speed regulating loop 48 contains the speed controller 80 and the current regulating loop 46, and functions to maintain the motor 18. speed at a value determined by the speed reference signal. The speed controller compares the speed feedback signal (from the tachometer to the speed reference (speed command) signal and generates an error signal which is supplied as a current reference or command signal to the current regulating loop 46 to produce the required value of armature current that will cause motor 18 to run at a speed where the speed feedback signal equals the speed reference signal.

Assume that forward rotation of motor 18 will rotate rolls 16 in the direction to drive strip 10 in the direction of arrow 12. Assume further, that the polarity of the speed reference 92 is positive for forward rotation of the motor, and that for forward motor rotation, the polarity of the speed feedback signal supplied through resistor 102 is negative. When the speed reference signal is greater than the speed feedback signal, the speed controller 80 is provided with a net positive input signal, and the controller 80 output increases in a negative direction, thus providing an increased negative command signal to the current controller 54. In turn, the output of controller 54 increases in a positive direction causing the power supply 52 to increase the current to armature 72 in the direction to increase the forward motor speed. The motor speed increases until the speed feedback signal from tachometer 100 equals the speed reference signal. The reverse occurs in case the speed feedback signal is larger than the speed reference signal. More specifically, if the speed feedback signal is greater than the speed reference signal, the net input to speed controller 80 is negative. As a result, the controller 80 output applies a less nega' tive command signal to the input of the current controller 54 thereby reducing the motor armature current. The motor speed will decrease until the speed feedback again equals the speed reference signal.

From the above it is seen that if it is desired to increase the motor speed, the output voltage of reference source 92 should be adjusted to a more positive reference value in order to supply a more positive speed command signal to the input of controller 80. Conversely, to reduce the speed, the output voltage of reference source 92 must be adjusted to a less positive value in order to supply a less positive speed reference signal to the input of the speed controller 80.

The swing roll position regulating loop 50 contains the speed regulating loop 48 and also includes a position error generating arrangement 108 for supplying a signal representing swing roll position to the input of the speed controller 80.

The position signal processing system 108 includes the controller 104 and a transducer 1'10 for generating an output signal that is a function of the swing roll 22 position. In the example shown, the reference or desired position of the swing roll is the mid point (dot-dash line 112) between the left and right excursion limits of the swing roll. The roll 22 is at the left limit when the pivot arm 24 engages stop 30, and at the right limit when the arm 24 engages stop 32. In the drawing the swing roll is shown at its left excursion limit. Although not restricted thereto, the arrangement disclosed herein is such that the swing roll 22 is limited to 30 excursions on each side of center in order to be compatible with the particular transducer arrangement hereinafter described by way of example.

Transducer may for example be an electromechanical type which responds to and translates the mechanical position of the swing roll into a DC voltage having a magnitude and polarity corresponding to the relative displacement of the swing roll from its reference position. Various forms of such transducers are known. The specific example shown at 110 includes a selsyn transmitter or rotatable transformer .114 with a rotor 116 mechanically coupled through a coupling 118 to the pivot arm 24 whereby the rotor of the selsyn'114 responds to the swing roll 22 and assumes a position corresponding to the position of the swing roll 22. The selsyn rotor 116 is connected to a suitable source of AC, while the conventional three windings of the selsyn stator 120 operating as secondaries are connected to the primaries of a pair of transformers 122 and 124. A pair of full-wave rectifiers 126 and 128 are connected across the respective secondaries of transformers 122 and 124. The DC outputs of rectifiers 126 and 128 are differentially connected across an output resistor 130 to provide thereacross an output voltage V3. The connection from the rectifiers is through equal-valued resistors 129 and 131.

The selsyn transmitter 114 is so related to the swing roll 22, that as the roll swings from the right hand limit to the left hand limit (stop 32 to stop 30), the rectifiers 128 output voltage V1 increases progressively from to some value, for example X volts while at the same time the output voltage V2 of rectifier 126 decreases from X volts to 0. Since the outputs of rectifiers 126 and 128 are oppositely poled across the output resistor 130, and, V1 and V2 being equal at their mutual crossover, the output voltage V3 across the output resistor 130 is 0 when the swing roll 22 is in the reference position (center line 112).

By way of example the circuit parameters of transducer 110 are so arranged that the output voltage V3 of transducer 110 progressively increases from +6 volts through 0 to 6 volts as the swing roll 22 moves from right to left (from stop 32 to stop 30). This is illustrated in FIG. 2, wherein the curve V3 represents the output voltage of the transducer 110. Since the swing roll in the example given moves 30 from center to right or left limit, the transducer output voltage V3 changes about 1 volt for each degrees. Thus, at swing roll positions 10", and left of center, the transducer output volttage V3 will be -2, 4 and -6, respectively. Similarly at swing roll positions 10, 20 and 30 right of center, the output voltage V3 will be +2, +4 and +6 respectively. The output voltage V3 appears on commonly connected output lines 132 and 134 of transducer 110.

Output line 132 of transducer 110 is connected to the input of controller 104 whose output line 136 is connectable through a switch 138 (when closed) to the input of the speed controller 80 where it is summed with the signals representing command speed and actual speed. Switch 138 is ganged for concurrent operation with a switch 140 in a later described circuit.

The system thus far described is a known prior art system which operates in the following manner. Assume that switches 138 and 140 are not ganged, i.e. they operate independently of one another. Assume further that switches 138 and 140 are open, and that for some reason so much slack has developed in the strip 10 in the section between roll sets 14 and '16 that the swing roll 22 is against the stop 30. Before the position regulating loop is switched ON, the human operator will adjust the pot 92 to increase the speed command signal in order to raise the speed of the strip 10- until the swing roll 22 is drawn to the reference position 112 (reference position in example is center). While the swing roll is at the center position, the human operator closes switch 138 (switch 140 remains open), thus changing the system operation from speed mode to position mode.

With the swing roll in center position, the transducer 110 output signal V3 on line 132 is 0. With 0 input to controller 104, its output on line 136 is also 0, and the speed controller 80 is unaffected by the position controller 104. Now assume that the paper manufacturing process is subjected to various conditions which tend to change the strip speed and/or tension. As such variations occur, swing roll 22 responds by changing position. The swing roll position is sensed by the selsyn 114, and the transducer 110 generates an output V3 of polarity and magnitude corresponding to the displacement of the swing roll from center position. This position error signal V3 is-fed along line 132 into the position controller 104 where the controllers integration characteristic (due to integrating capacity) causes the output of position controller 104 to increase as long as position error persists. The output of position controller 104 is applied along line 136 to the speed controller 80, where it either adds or substracts from the command speed signal coming from the reference source 92. The corrective signal from controller 104 causes the speed regulating loop 48 to regulate for a new speed until the swing roll has been returned to its center position.

In the immediately preceding operational example, in order to avoid damaging transients, it was necessary for the operator to move the swing roll to center position before switching the system from speed to position mode. In order to save time, or to reduce operator error, or for other reasons, it is desirable to be able to switch from speed to position modes regardless of the then position of the swing roll 22, even though it may be against one of the limit stops. The system thus far described is unable to do this. However, in accordance with the present invention a shift from speed to position mode may be effected without introducing severe transients even with the swing roll at either of its excursion limits, by combining with the aforedescribed system a position signal limit control arrangement operative in response to swing roll position, to initially limit to a relatively low value the magnitude of the swing roll position signal applied to the input of the speed controller, and to remove that limitation in response to the swing roll reaching a position within a small bandwidth of the center posi.ion. An example of such a limit control scheme is shown at 142. i

The limit control system 142 includes any suitable signal-response adjustable limiter 144 for limiting the output of controller 104 to selectable limits. By way of example, the limiter 144 is shown as a voltage-responsive feedback type limiter such as is described and claimed in copending US. patent application Ser. No. 508,224, entitled Voltage Adjustable Limiter Elements, and filed Nov. 17, 1965, by Robert E. Hull. Also included in the limit control system 142 are voltage reference sources 146 and 148 and a switching system 150 responsive to the output of transducer and thereby to the position of the swing roll 22. The switching system selectively applies either of two different reference voltage values of both polarities to limiter 144 to set relatively low or relatively high limits to both positive and negative output of the controller 104.

The positive reference source 146 is shown as a voltage divider connected across a suitable voltage source and having taps 152 and 154 selectively connectable by a relay 156 to the positive reference input line 158 of limiter 144. Relay 156 is provided with normally open contacts 160 and normally closed contacts 162. Normally open and normally closed refer to contact status when the operating coil of the associated relay is unenergized. This convention is followed in connection with all electromagnetic relays shown herein. With relay 156 unenergized, contacts 162 connect the low voltage positive reference tap 154 to the positive reference line 158 thereby to limit the positive output of controller 104 to a very small value. On the other hand, when the relay 156 is energized, contacts 160 connect the high positive voltage reference tap 152 to the positive reference line 158 thereby setting the positive output limit of amplifier 104 at a relatively high value, thus effectively removing the low positive limitation. In the same manner high and low negative voltage taps 164 and 166 of the reference source 148 are selectively connectable through contacts 168 and 170 of a relay 172 to the negative reference line 173 of limiter 144. Relays 156 and 172 are shown unenergized. The low value to which the limiter 144 restricts the output of controller 104 may be selected by adjusting taps 154 and 166.

With switch 140 closed, relays 156 and 172 are initially energized in response to concurrent closure of contacts 174 and 176 of a pair of voltage sensitive relays 178 and 180, which closure also energizes a relay 182 whose contacts 184 close a circuit to seal relays 156 and 172 in the energized state. Relays 178 and 180 are responsive to the transducer 110 output V3 on line 134, which is connected to input lines 186 and 188 connected to the volt sensitive relays 178 and 180 respectively.

By way of example and to be compatible with the hereinbefore discussed examples of transducer output V3 values (chart FIG. 2), relay 178 is arranged to open contacts 174 when the transducer output voltage V3 exceeds 1 volt in the negative direction, and to close contacts 174 when the voltage V3 exceeds -1 volt in the positive direction. Similarly, relay 180 is arranged to close contacts 176 when the voltage V3 on line 134 is less than +1 volt, and to open contacts 176 when V3 exceeds +1 volt.

Although other suitable voltage sensitive relays may be used, relays 178 and 180 may for example be of the type described and claimed in copending US. patent application Ser. No. 379,983, filed July 2, 1964, and entitled Amplifying Apparatus Providing Two Output States, wherein a voltage-sensitive bistable amplifier drives the operating coil of an electromagnetic relay. In relay 178, a bistable amplifier 190 drives the operating coil of a relay 192 associated with normally closed contacts 174. Similarly in relay 180, a bistable amplifier 194 drives the operating coil of a relay 196 associated with normally open contacts 176. 7

By way of example, each of amplifiers 190 and 194 is arranged to energize (pull in) the respective relay connected to its output only in response to a net negative signal of say at least 0.1 volt (-0.1 volt or more negative), and to release (drop out) the relay in response to a net input of 0.05 or less negative. For relay 178 to open contacts 174 when transducer output V3 on line 134 exceeds 1 volt negatively, an input bias of approximately +1 volt is applied to the input of amplifier 190 from a bias source 198. Likewise in order to close contacts 176 in response to voltage V3 being below +1 volt, an input bias of 1 volt is applied to the amplifier 194 from a bias source 200.

The right side of switch 140 is connected to a source of power, while the left side of this switch is connected through a line 202 to one side of contacts 174 and to one side of contacts 184. The other side of contacts 174 are connected through a line 204 to one side of contacts 176, whose other side is connected to the other side of contacts 184 and through a line 206 to the operating coils of relays 182, 156 and 172. It should be apparent that when switch 140 and contacts 174 and 176 are closed, relays 182, 156 and 172 will be energized. When relay 182 is energized, contacts 184 are closed to seal or latch the relays 182, 156 and 172 in the energized state.

A further understanding of the invention may be had from the following examples of operation. Assume that all the components of the circuit are in the positions shown'in FIG. 1. Switches 138 and 140 are open, and motor 18 is rotating forwardly, driving strip 10 in the direction of arrow 12 at a regulated speed dictated by the value of the command speed signal from reference source 92. For some reason there is enough slack in the strip so that the air loading on pivot arm 24 has forced the swing roll 22 to its extreme left position (against the stop 30). In response to this position of the swing roll, the output V3 of the transducer 110 is at 6 volts.

Because of the +1 volt input bias applied to amplifier 190, the 6 volts on line 134 results in a net input of -5 volts to amplifier 190, thus energizing relay 192. Since the input bias to amplifier 194 is 1 volt, the

added input -6 vol-ts results in a net input of 7 volts to amplifier 194. Thus relay 196 is also in the energized condition. Relays 192 and 196 are illustrated in the energized mode with contacts 174 open and contacts 176 closed. Relays 182, 156 and 172 are therefore unenergized as shown.

Under these circumstances, the positive and negative low value reference voltages from taps 154 and 166 are applied through closed contacts 162 and 170 respectively to the limiter 144, and as a result the output of controller 104 on line 136 is limited to a very low value dictated by the particular value of reference voltage applied along lines 158 and 173 to the limiter 144.

Now assume that ganged switches 138 and 140 are manually closed to switch the system from the speed mode to the position mode. The limited value position error signal on line 136 is applied to the input of speed controller 80. This small valued position error signal is of positive polarity and is just enough to modify the speed command to the speed controller in such a direction as to slowly increase the speed of motor 18 and of strip 10 to slowly and smoothly pull the swing roll 22 back towards its reference (center) position.

When the swing roll, in its travel from left toward center, just passes the 5 position left of center, the transducer output V3 becomes more positive than -1 volt and relay 192 is released to close contacts 174. Relays 182, 156 and 172, are thereby energized thus disabling the low limit circuit and enabling the high limit circuit. As a result, the limitation to a low value of the output of controller 104 is removed, and a new limit of a relatively high value is placed on the output of controller 104. The output of controller 104 now responds fully to and is representative of the swing roll position, because varying magnitudes in the output of controller 104 that are fully representative of the Swing roll position will be below the high valued magnitude to which the output of controller 104 is restricted or limited by connection of the high limit reference taps 152 and 164 to lines 158 and 173 of the limiter 144. This allows controller 104 to operate within normal operating output values and permits normal operation of the swing roll position regulating loop 50 wherein the output of the position controller 104 supplies a more truly representative swing roll position signal to the input of speed controller 80 where it either adds or subtracts from the command speed reference supplied from source 92. The corrective signal from controller 104 causes the speed regulating loop 48 to regulate for a new speed until the swing roll has been returned to its center position from any deviation thereof.

It should be noted that relay 182 being energized, relays 182, 156 and 172 are latched in the energized state by the connection of line 202 through closed contacts 184 to line 206. Thus until switch 140 is opened, relays 156 and 172 will remain energized regardless of the action of relays 178 and 180.

In a second operational example, assume that switches 138 and 140 are open, and the strip 10 is so taut that the swing roll 22 is pulled up against the right stop 32. Under these conditions, the output of transducer will be +6 volts, and relays 192 and 196 will be unenergized, whereby contacts 174 will be closed and contacts 176 will be open. Relays 182, 156 and 172 will be unenergized, and the controller 104 output from line 136 will be limited to a low value. Now when switches 138 and are manually closed, the output on line 136 will be of negative polarity and just enough magnitude to cause the speed regulating loop 48 to call for a slow reduction in speed of the strip 10, causing swing roll 22 to slowly swing leftward toward the center position 112. In its travel from right to left toward center, just as the swing roll passes the 5 right position, the transducer 110 output V3 falls below +1 volt thereby energizing relay 196 to close contacts 176. It should be recalled that contacts 174 are in a closed position. As a result, relays 182, 156 and 172 will be energized to recalibrate the output limit on controller 104 from a low value to a high value, thereby placing the system in normal operation. Again relays 156 and 172 are latched in the energized mode to maintain normal system operation.

It should be noted from the fore-going description, that relays 178 and 180 operate to recalibrate the output limits of controller 104 from low to high when the swing roll, in moving toward center from either direction crosses into a zone of small bandwidth extending, in the example given, about on each side of the reference (center) position. It should also be apparent that initially the full impact of a wide position error is limited to provide a soft transition from speed to position modes. Also since the recalibration from low to high output limits is made in a small zone near center, the full output of the transducer is low because the real error is small, and the uninhibited corrective signal from controller 104 to the speed regulator is so small that correction of speed is made withou severe transients.

It may be noted that the limit placed on the output of controller 104 by the high limit reference signals when relays 156 and 172 are energized, is only for the purpose of limiting the output to safe capabilities of the apparatus. If such limitation is not needed, then all that relays 156 and 172 have to do when energized, is to open contacts 162 and 170 in order to remove the low limitation placed on the controller 104 output. In such case, contacts 160 and 168 are not required.

It is to be understood that the herein described arrangements are simply illustrative of the principles of the invention, and that other embodiments and applications are within the spirit and scope of the invention.

What is claimed is:

1. In a system for regulating to a reference first position a movable member that is in contact with a traveling strip driven by a motor whose speed is regulated by speed controlling first means in response to the difference between actual strip speed and a reference strip speed the latter as modified by error in the position of said member, and wherein said member is movable through a range having a section beginning with and extendmg from said first position to a second position representlng substantial deviation from said first position, position error generating second means responsive to the position of said member for producing as an output a posltron error signal which is a function of the position of said member, third means for applying said position error signal to said speed controller, fourth means for limiting the output of said error generating means to less than it would otherwise be for the error generating means over a portion of said section which portion extends to said second position, whereby the output of said error generating means is initially applied to said speed control means wlthout severe transients even if said member be as far away from said first position as said second position, and fifth means for removing said limitation on the output of the error generating means in response to said member reaching a selected point in said section while moving in a direction away from said second position and toward said first position.

2. The combination as in claim 1 wherein said member is a tension member resiliently biased toward said stri 3 The combination as in claim 1 wherein said second means includes an amplifier, said fourth means includes a limiter circuit connected to the amplifier for limiting the output of said amplifier, and wherein said fifth means includes means for rendering ineffective said limiter circuit.

4. The combination as in claim 1 and further including sixth means responsive to said member reaching said selected point while moving in said direction for latching said fifth means in a limit removing state and maintaining said state thereafter regardless of the position of said member.

5. The combination as in claim 4 and further including common control means coupled to said third means and to said sixth means, said common control means being selectively operable in first and second modes, said first mode concurrently rendering both said third and sixth means ineffective, said second mode concurrently rendering both said third and sixth means effective.

6. The combination as in claim 4 wherein said second means includes an amplifier, said fourth means includes a limiter circuit connected to the amplifier for limiting the output of said amplifier, and wherein said fifth means includes means for rendering ineffective said limiter circuit.

7. The combination as in claim 1 wherein said fourth means limits the output of said error generating means to a fixed value.

8. The combination as in claim 7 wherein said second means includes an amplifier, said fourth means includes a limiter circuit connected to the amplifier for limiting the output of said amplifier, and wherein said fifth means includes means for rendering ineffective said limiter circuit.

9. The combination as in claim 7 and further including sixth means responsive to said member reaching said selected point while moving in said direction for latching said fifth means in a limit removing state and maintaining said state thereafter regardless of the position of said member.

10. The combination as in claim 9 and further including common control means coupled to said third means and to said sixth means, said common control means being selectively operable in first and scond modes, said first mode concurrently rendering both said third and sixth means ineffective, said second mode concurrently rendering both said third and sixth means effective.

11. The combination of claim 1 wherein said range includes a second section beginning with and extending from said first position to a third position in a direction opposite to said second position, whereby said first position is between said second and third positions, and wherein there is sixth means for limiting the output of said error generating means to less than it would otherwise be for the error generating means over a portion of said second section which portion extends to said third position, and wherein there is seventh means for removing said limitation imposed by said sixth means in response to said member reaching a selected point in said second section while moving in a direction away from said third position and toward said first position.

12. The combination as in claim 11 wherein said member is a tension member resiliently biased toward said strip.

13. The combination as in claim 11 wherein said limitation imposed by said fourth means is a fixed value, and said limitation imposed by said sixth means is a fixed value.

14. The combination as in claim 11 which further includes eighth means for latching said fifth means in a limit removing state and maintaining that state thereafter regardless of the position of said member in response to said member reaching said selected point in the first said section while moving in a direction away from said second position and toward said first position, and for latching said seventh means in a limit removing state and maintaining that state thereafter regardless of the position of said member in response to said member reaching said selected point in said second section while moving in a direction away from said third position and toward said first position.

15. The combination as in claim 13 which further includes eighth means for latching said fifth means in a limit removing state and maintaining that state thereafter regardless of the position of said member in response to said member reaching said selected point in the first said section while moving in a direction away from said sec- 0nd position and toward said first position, and for latching said seventh means in a limit removing state and maintaining that state thereafter regardless of the position of said member in response to said member reaching said selected point in said second section while moving in a direction away from said third position and toward said first position.

16. The combination as in claim 14 which further includes common control means coupled to said third means and said eighth means for selectively and concurrently rendering both said third and eighth means inelfective or both effective as desired.

17. The combination as in claim 15 which further includes common control means coupled to said third means and said eighth means for selectively and concurrently rendering both said third and eighth means ineffective or both effective as desired.

References Cited UNITED STATES PATENTS 2,419,812 4/1947 Bedford 3l830 X 2,474,620 6/1949 Fath 318-6 X 3,206,663 9/1965 Neal et a1. 318-28 3,324,363 6/1967 Hill et a1. 3186 ORIS L. RADER, Primary Examiner.

-B. A. COOPER, Assistant Examiner.

US. Cl. X.R. 

